Moisture indicator and method of making the same



United States Patent 3,173,880 MOISTURE INDICATOR AND METHOD OF MAKING THE SAME William S. Pappas and Charles W. Weber, Oak Ridge, Temp, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed July 13, 1961, Ser. No. 124,310 7 Claims. (Cl. 252-408) Our invention relates to a moisture indicator and more particularly to an indicator for determining the moisture content of liquids of low water solubility and to a method of preparing the same.

The presence of moisture presents a serious problem in conjunction with certain uses of liquids of low water solubility, for example, in halogenated hydrocarbon cooling and refrigeration systems. Even a relatively minute amount of water, e.g., as low as 20 parts per million in the coolant available commercially under the trade name Freon 114 (dichlorotetrafluroethane), may result in excessive corrosion of cooling systems, and particularly of aluminum system components. Small quantities of water may be present in the coolant as obtained commercially or may enter the system through faulty seals or other leaks. Adequate means are available for removal of this water from the coolant; for example, the coolant stream may be contacted with desiccant means such as molecular sieves. A continuing need has existed, however, for a simple, reliable indicator which would clearly indicate the coolant moisture content in a manner intelligible to relatively unskilled personnel.

Various indicators have been employed previously for determining the moisture content of liquids of low water solubility, but none have proved entirely satisfactory, particularly for halogenated hydrocarbon coolant systems requiring a moisture content below 20 parts per mililon. One type of indicator utilizes cobalt bromide incorporated onto a cellulosic or glass-like fiber as the active material, with the indicator changing its color as the moisture reaches certain levels. The color changes produced in this type indicator, however, are complex and indistinct, with an elaborate color reference chart being required for determining the exact color and the moisture content which produced the color. Another type of indicator employs silica gel impregnated with cobalt bromide and subsequently heat-treated. This type indicator, while useful for indicating the relative humidity of gaseous mixtures such as air, is inadequate for liquids of low water solubility such as halogenated hydrocarbons because the color shift occurs gradually and the color changes are indistinct gradations of a single basic blue color.

It is particularly desired to provide an indicator which changes color sharply and distinctly at a moisture content within the range of 10 to 20 parts per million and preferably at approximately parts per million in Freon 114 at a temperature of approximately 110 F. Certain large-scale cooling systems operate at an ambient temperature of 110 F., and the moisture content is most conveniently determined at this temperature. In order to provide continuous monitoring of the coolant moisture content an indicator suitable for continuous Contact with a portion of the coolant stream is desired.

It is, therefore, an object of our invention to provide an indicator for determining the water content of liquids of low water solubility.

Another object is to provide an indicator for determing the water content of halogenated hydrocarbons.

Another object is to provide an indicator which undergoes a distinct color change at a halogenated hydrocarbon water content of 10 to parts per million.

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Another object is to provide an indicator which undergoes a distinct color change at a dichlorotetrafluoroefliane water content of approximately 15 parts per million at a temperature of approximately F.

Another object is to provide a method of preparing an indicator to accomplish the aforesaid objects.

Other objects and advantages of our invention will be apparent from the following detailed description and claims appended hereto.

In accordance with our invention a moisture indicator for liquids of low water solubility is prepared by impregnating activated silica gel with approximately 2 to 3 weight percent cobaltous bromide and at least 0.5 weight percent hydrobromic acid. The impregnated gel changes color distinctly and reversibly at the moisture content desired, i.e., within the range of 10 to 20 parts per million in Freon 114 at 110 F. The color clearly shifts from pink to green as the moisture level falls below approximately 15 parts per million in this medium and back to pink as the moisture rises above this level. Although the color shifts sharply within a narrow range of less than 3 parts per million, the accuracy of determination of the exact level at which the shift occurs is limited by the accuracy of the analytical method used, e.g., :5 parts per million for Karl Fischer titration. By disposing this indicating material in a suitable container and passing a portion of the coolant stream through the container, the presence or absence or" this amount of moisture may be continuously indicated.

We have found that the addition of a small amount of hydrobromic acid to a cobaltous bromide-silica gel indicator results in a readily visible color shift which occurs sharply within a narrow moisture range. The reason for this improved color change is not understood, although complex chemical reactions of the hydrobromic acid with the other constituents employed in the preparation of the indicator are apparently involved. The composition of the chemical specie obtained by impregnating the silica gel with cobaltous bromide and hydrobromic acid is not known.

The silica gel is readily impregnated with cobaltous bromide and hydrobromic acid by preparing a solution of the latter ingredients in sufficient water to wet the silica gel completely and contacting the solution with the gel whereby the solution is absorbed. The gel is then dried, preferably by contacting the gel with dry air at room temperature (20 C. to 30 (3.). No heat treatment is required.

The proportion of cobaltous bromide to silica gel is critical to obtaining an indicator with the desired prop erties. Cobaltous bromide within the range of approximately 2 to 3 weight percent of the silica gel may be employed and approximately 2.3 weight percent is preferred. These proportions are .based upon the use of cobaltous bromide in its hydrated form CoBr -6H O as the starting material. Silica gel is supplied in the form of activated silica gel commercially available as an adsorbent.

At least approximately 0.5 weight percent hydrobromic acid with respect to the Weight of silica gel is required, with the upper limit of this constituent not being critical since the excess P181" is removed in the drying step. In order to ensure the presence of a sufiicient amount, approximately 0.7 weight percent HBr is preferred. Hydrobromic acid may be conveniently supplied in the form of a commonly available 48 weight percent solution. In handling HBr in the form of an aqueous solution, the solution may become contaminated with small amounts of free bromine as evidenced by brown coloration of the solution. The presence of such free bromine is not critical and has no adverse affect on the properties of the the indicator.

The active indicatin material prepared as described above is utilized by disposing the material in a suitable container provided with visual indication means such as a sight glass and introducing a sample of the liquid in quesion. In a circulating system continuous indication of the liquid moisture content may be obtained by continuously passing a portion of the circulating stream through the active material. Although not critical, it is preferred in the case of coolant systems to continuously withdraw a portion of coolant stream in the vapor state, condense the vapor to the liquid state by means of a conventional cooling coil and pass the condensate through the indicator, after which the condensate is returned to the coolant stream. For Freon 114, cooling of the condensate to a temperature of approximately 110 F. is preferred.

The indicator prepared by the method of our invention is applicable generally to liquids of low water solubility, that is, liquids in which the water concentration a saturation is less than approximately 5000 parts per million. In addition to Freon 114, the presence or absence of a particular moisture content may be determined for other halogenated hydrocarbons such as chloropentaiiuonoethane and trichlorotrifluoroethane by his means. Other examples of applicable liquids of low water solubility are organic solvents and petroleum fractions such as gasoline and kerosene. The particular moisture revel at which the indicator color shift occurs for these liquids may be determined by conventional analysis.

The moisture level at which the indicator color shift occurs varies with the temperature. Accordingly, measurement at a constant temperature is required in order to obtain accurate results. For Freon 114 a temperature of 90 to 120 F. may be employed to obtain a color shift within the range of to parts per million of moisture and a temperature of approximately 110 F. is preferred.

Our invention is further illustrated by the following specific example.

Example Active indicator material was prepared by the following procedure: 980 milliliters of water, grams of CoBr -6H O and 20 milliliters of 48 weight percent aqueous hydrobromic acid solution were combined, and 1500 grams of activated silica gel was wetted with the resulting solution. The wetted gel was then dried at room temperature (24 C.) with dry air. Approximately one gram of the dried gel was disposed in an indicator sight tube comprising a metal container provided with a sight glass and an inlet and outlet. sate containing varying amounts of moisture at a temperature of 110 F. was then passed through the sight tube and the indicator color was visually observed. Thirteen samples of the condensate Were Withdrawn and analyzed for moisture content by means of Karl Fischer titration. Six of the samples produced a green (dry) color and had the following moisture content in parts per million: 6, 14, 19, 15, 9 and 11. The remaining seven samples produced a pink (wet) color and had the following moisture content in parts per million: 20, 21, 22, 23, 17, 36 and 48.

It may be seen from the above that the presence or absence of moisture in Freon 114 at a level of 10 to 20 parts per million may be accurately determined by this means. All of the samples which produced a dry color had less than 20 parts per million of moisture, and all of the samples which produced a wet color had a moisture content of 20 parts per million or more except one hav- Freon 114 conden- 4. ing 17. The slight overlapping in results is attributed to the accuracy (i5 parts per million) of the Karl Fischer analytical method.

The above example is merely illustrative and is not to be construed as limiting in any manner the scope of our invention, which is limited only as indicated by the appended claims.

Having thus described our invention, we claim:

1. The method or" preparing a moisture indicator which comprises wetting activated silica gel with an aqueous solution containing cobaltous bromide within the range of approximately 2 to 3 percent by weight of said silica gel and hydrobromic acid in an amount of at least approximately 05 percent by weight of said silica gel and drying the resulting wetted gel.

2. The method of claim 1 in which the amount of cobaltous bromide is approximately 2.3 percent by weight of said silica gel.

3. The method of claim 1 in which the amount of hydrbrornic acid is approximately 0.7 percent by weight of said silica gel.

4. Indicating material for a moisture indicator consisting of activated silica gel impregnated with approximately 2 to 3 weight percent cobaltous bromide and at least approximately 0.5 weight percent hydrobromic acid.

5. The material of claim 4 in which the silica gel is impregnated with cobaltous bromide in an amount equivalent to approximately 2.3 percent by weight of said silica gel.

6. The material of claim 4 in which the silica gel is impregnated with hydrobromic acid in an amount equivalent to approximately 0.7 percent by Weight of said silica gel.

7. Indicating material for determining the moisture content of dichlorotetrafluoroethane consisting of activated silica gel impregnated with approximately 2 to 3 weight percent cobaltous bromide and at least 0.5 Weight percent hydrobromic acid, said silica gel being ink in color when exposed to dichlorotetrafiuoroethane containing more than approximately 15 parts per million of moisture at a temperature of F. to F. and green in color when exposed to dichlorotetrafluoroethane containing less than approximately 15 parts per million of moisture at said temperature.

References Cited by the Examiner UNITED STATES PATENTS 1,825,896 10/31 Bond 23-230 2,362,396 11/44 Parks 23-230 2,418,033 3/47 Kamlet 252-408 2,761,312 9/56 Line et a1 73-53 2,800,460 7/57 Grosskopf 252-408 2,836,974 6/58 Krause 252-408 XR 2,844,026 7/58 Wischmeyer et a1. 73-53 2,848,421 8/58 De Forest 252-408 OTHER REFERENCES E. I. du Pont de Nemours & (30., Wilmington, Del., pages 2, 5 relied on.

Freon Technical Bulletin, B-2, 1962.

ALBERT T. MEYERS, Primary Examiner.

ANTHONY SCIAMANNA, Examiner. 

1. THE METHOD OF PREPARING A MOISTURE INDICTOR WHICH COMPRISES WETTING ACTIVATED SILICA GEL WITH AN AQUEOUS SOLUTION CONTAINING COLBALTOUS BROMIDE WITHIN THE RANGE OF APROXIMATELY 2 TO 3 PERCENT BY WEIGHT OF SAID SILICA GEL AND HYDROBROMIC ACID IN AN AMOUNT OF AT LEAST APPROXIMATELY 0.5 PERCENT BY WEIGHT OF SAID SILICA GEL AND DRYING THE RESULTING WETTED GEL.
 7. INDICATING MATERIAL FOR DETERMINING THE MOISTURE CONTENT OF DICHLOROTETRAFLUOROETHANE CONSISTING OF ACTIVATED SILICA GEL IMPREGNATED WITH APPROXIMATERLY 2 TO 3 WEIGHT PERCENT COBALTOUS BROMIDE AND AT LEAST 0.5 WEIGHT PERCENT HYDROBROMIC ACID, SAID SILICAA GEL BEING PIINK IN COLOR WHEN EXPOSED TO DICHLOROTETRAFLUOROETHANE CONTAINING MORE THAN APPROXIMATELY 15 PARTS PER MILLION OF MOISTURE AT A TEMPERATURE OF 90*F. TO 120*F. AND GREEN IN COLOR WHEN EXPOSED TO DICHLOROTETRAFLUOROETHANE CONTAINING LESS THAN APPROXIMATELY 15 PARTS PER MILLION OF MOISTURE AT SAID TEMPERATURE. 